1. Field of the Invention
The present invention relates to a method of and a device for checking finished cigarettes or other similar products, notably filter-tip cigarettes, in order to detect and discard defective cigarettes.
The term "defective cigarettes" means cigarettes of which the envelope is leaking for example on account of a faulty joint, a not properly fitted sleeve, a tear, a not properly glued seam, etc., or alternatively cigarettes insufficiently filled with tobacco, such as cigarettes having an empty end portion or pocket.
Considering the very high operating speeds of modern cigarette making machines, this checking operation cannot be performmed manually and therefore some automatic means must be provided for checking the cigarettes separately along the continuous stream of cigarettes produced by the machine, so as to discard faulty cigarettes and allow only sound cigarettes to proceed along the production circuit.
2. Description of the Prior Art
Many methods and devices have already been proposed for performing this checking operation automatically, notably by blowing air at predetermined pressure flow rate values through the cigarette, and measuring the pressure of flow rate of the air escaping from the opposite end of the cigarette, or alternatively by sucking or compressing air from or into a chamber surrounding the cigarette and measuring the pressure existing within the cigarette.
Among the various known methods devised for checking cigarettes, one method consists in transferring continuously across their longitudinal axes a series of regularly spaced cigarettes, causing said cigarettes to pass through a checking station, introducing gas from a suitable source of compressed gas into each cigarette passing through said control station, the compressed gas being delivered into a chamber communicating with one end of the cigarette while the opposite end thereof is momentarily sealed, measuring the pressure of the gas thus introduced into the cigarette, delivering a signal corresponding to the measured pressure, comparing said signal with a predetermined threshold value representative of a minimum pressure corresponding to an acceptable cigarette, and generating a rejection signal when the instantaneous value of the measured pressure is lower than said minimum pressure. In known devices for carrying out this prior art method, said chamber is supplied permanently with gaseous fluid, notably compressed air, and communicates with the external atmosphere during the time periods in which it is not connected to the end of a cigarette. When said chamber is connected to the cigarette end, the communication between said chamber and the atmosphere is discontinued or cut off, so that the pressure rises in the chamber. Theoretically, if the cigarette is not defective, the pressure in the chamber rises above the aforementioned minimum pressure and no rejection signal is generated.
However, in actual practice it appeared that in many cases these known checking devices eject a number of cigarettes without the certainty that the ejected cigarettes are really defective, or that all the defective cigarettes are effectively rejected. Although the reasons of this misfunctioning are not completely obvious, one may think that one reason is that the leakage flow of the gaseous fluid through the defective cigarette is in most instances extremely low. Since the chamber is supplied permanently with gaseous fluid, this continuous supply appears to compensate one fraction, if not the whole, of the leakage flow through the defective cigarette, so that the fluid pressure in said chamber may rise to a value higher than the above-mentioned minimum pressure, and thus the defective cigarette is not rejected. This obviously impairs the sensitivity and therefore the reliability of the cigarette checking device.
Moreover, in hitherto known cigarette checking devices of the above-mentioned type the pressure measured in the chamber is converted into an electric signal of which the value varies with the pressure and is compared with said predetermined threshold value corresponding to the aforesaid minimum pressure. The comparator provided for this purpose delivers an output signal only when the measured pressure exceeds said minimum pressure. The output signal of said comparator will thus indicate that the cigarette being checked is not defective, and therefore said signal cannot be utilized directly for controlling the means for rejecting or ejecting the faulty cigarettes. Consequently, the device must comprise on the one hand a synchronization device capable of generating a series of pulses, by reason of one pulse per cigarette, and on the other hand a logic circuit to which the output signal of said comparator and said series of pulses are fed for delivering at the output of said logic circuit a rejection signal when, within a predetermined time period, not output signal frm said comparator is delivered to said logic circuit. Therefore, the faulty cigarette rejection control logic circuit is relatively complicated insofar as it requires a preliminary treatment of the comparator output signal.